The field of the present invention relates generally to power switching circuits and devices, and more specifically to such circuits and devices, including the combination of bipolar and field effect transistors.
It is known in the prior art to apply metal oxide semiconductors (MOS devices) for use in low power digital circuits, and to apply power bipolar transistors, bipolar Darlington circuits, and silicon-controlled-rectifiers (hereinafter referred to as SCRs) high-power applications. Bipolar transistors operated near their rated current and voltage are susceptible to damage from current transients, and have a positive temperature coefficient of current, which may lead to "thermal runaway" when such devices are connected in parallel, as certain areas of the bipolar transistor's substrate heat up under severe operating conditions, causing damage or destruction of the transistor(s). Bipolar power switching transistors typically have DC input impedances ranging from 1.0 to 10.0 ohms, and DC current gains from 10 to 50. Darlington circuits typically have a high saturation voltage across their main current carrying path, resulting in a relatively high static power dissipation. SCRs are widely applied for use in power circuits, but have the disadvantage that once they are turned on via a signal applied to their gate electrode, they cannot be turned off by applying another signal to, or removing the original signal from, their gate electrode. SCRs are turned off by either substantially reducing toward zero magnitude the current flowing through their main current path, or by reducing to zero value the voltage across their anode and cathode electrodes. Accordingly, in many applications where the voltage across an SCR's anode and cathode electrodes does not "naturally" reduce to zero, complicated circuitry is required to turn off the SCR. Accordingly, from the above discussion it is apparent that a present problem in the current state of the art in power switching technology, is to provide a relatively high power switching device or circuit that has (1) relatively high switching speeds, (2) relatively low static power dissipation, (3) immunity to thermal runaway, (4) good transient current capability, and (5) can be turned on or off via the application of low power signal to its control electrode.